This invention relates generally to temperature sensing devices and more particularly concerns temperature sensing devices usable for thermal switching.
A typical thermocouple is illustrated in FIG. 1. It consists of a positive thermocouple wire 11 connected to a negative thermocouple wire 13 at a measuring junction 15. The wires 11 and 13 are of dissimilar metals so that current will flow through the wires 11 and 13 when the junction 15 is heated. The voltage across the wires is a function of the composition of the two wires 11 and 13 and the temperature applied to the junction 15. This is generally known as the Seebeck effect and the voltage across the two wires referred to as the Seebeck voltage in reference to Thomas Seebeck who discovered the phenomena in 1821. Thermocouples for measuring the Seebeck voltage, such as the thermocouple illustrated in FIG. 1, might typically employ a copper positive wire 11 and a constantan negative wire 13 separated within an insulation material 17 such as magnesium oxide, all encased within a protective sheath 19.
There are several problems inherent in the type of thermocouple above described. The positive and negative wires 11 and 13 of known thermocouples are generally quite fragile, typically of sixteen gauge wire or finer, and consequently have a limited life in most applications. A life of approximately one year is quite common. The cost of manufacture of known thermocouples is increased due to the need for an internal weld at the measuring junction 15. While these problems may have been unavoidable in thermocouples used in temperature transducer applications where a linear relationship between the junction temperature and the Seebeck voltage is required, there are many switching applications for which such linearity is unnecessary. However, thermocouples such as that hereinbefore described are presently used for both transducer and thermal switching applications.
It is, therefore, a primary object of this invention to provide a millivoltage generator employing the Seebeck effect usable in thermal switching applications. It is further an object of this invention to provide a millivoltage generator having a greater life expectancy than known thermocouples. Another object of this invention is to provide a millivoltage generator which requires no internal weld. It is also an object of this invention to provide a millivoltage generator which is less expensive than known thermocouples. And, it is an object of this invention to provide a millivoltage generator in which the generator sheath replaces one of the wires used in known thermocouples.